Many tanks and pipes are of double wall construction. The inner wall provides primary containment and the outer wall provides secondary containment. The space between the walls is referred to equivalently as the annular space, annulus, interstitial space, interstice or secondary containment space.
Various approaches of leak detection methods have been applied to testing the integrity of primary and/or secondary walls. The interstitial space can be put under pressure or vacuum and the pressure may be monitored as an indicator of leakage. Further, the interstitial space can be monitored for liquids that might leak from the primary containment. Still further, the interstice can be filled with liquid and the liquid level monitored as an indicator of leakage.
However, there are shortcomings to each known approach. For example, the current practices of testing the integrity of the interstitial space using pressure or vacuum decay are not able detect leaks as small as of 0.005 cc/sec (or gal/h).
The practice of monitoring the interstitial space for the presence of a liquid does not evaluate the integrity of the secondary wall (unless the tank is submerged in liquid). Additionally, the containment must be filled with liquid before monitoring can begin. If the secondary containment is leaking, fluid may be released before reaching the interstitial liquid monitor.
It is also known to add a unique chemical tracer to one side of a barrier and to test and/or monitor for the tracer on the other side of the barrier. Conceptually, a tracer or vapor mixture could be added to the primary containment, and detected in the interstice and/or the tracer could be added to the interstice and detected inside the primary containment and/or outside the secondary containment. However, in practice, the tracer must diffuse, a relatively slow process over significant distances, or be transported by fluid flow or convection between any possible leak location and the sampling location.
Further, it is known in manufacturing processes to introduce a tracer chemical into a primary container and test for a tracer chemical in a secondary container. The secondary container may be flushed or evacuated in order to transport any tracer released by a leak to the detector. However, in practice, this method involves containers of limited size (less the 200 liters) for the purpose of leak detection. In addition, the distances involved usually allow for rapid transport of the tracer to the detector. Such methods are not scalable for larger containers necessarily having large dimensions and therefore larger distances (more than a few feet), such as, for example, large double wall containers (greater than about 1000 liters) that may be built with large distances between the leak location and a single interstitial access.
It is also known to add a tracer to the annular space of a double wall container surrounding the primary chamber and test for the tracer inside a primary chamber. The tracer mixture is added to one end of the annular space and flushed through to an opening at the other end until the tracer mixture is present throughout the annulus. The pressure differential between the annulus and the primary chamber causes fluid to flow from the annulus to the primary chamber in the event of a leak.
It is known to rely on diffusion to transport the tracer throughout the interstitial space. If a tracer is added to the primary containment and the primary containment is pressurized, any fluid which leaks into the interstice will be transported into the interstice. The period between the addition of the tracer to the primary containment and the collection of the test sample is dictated by the rate of diffusion of the tracer and the distance.
The addition of tracer to the annulus and the subsequent distribution of the tracer may also rely on diffusion, but the time requirement is greater than for detection of leakage of the tracer to the interstice, because the tracer mixture in the interstice must relatively evenly distributed throughout the annulus.
A reliable leak detection method for larger containers with double walls, wherein even low-level leaks (as small as 0.005 cc/sec) are detected quickly, would be highly advantageous.